Vehicle Location and Navigation Systems based on LEDs Grantham Pang, Hugh Liu, Chi-Ho Chan, Thomas Kwan Dept. of Electrical and Electronic Engineering The University of Hong Kong Pokfulam Road Hong Kong Fax : (852)-2559-8738 Tel: (852)-2857-8492 Email:
[email protected]
SUMMARY In the future, the traffic lights, traffic signal devices, message display boards could all be replaced by Light Emitting Diodes (LEDs). Therefore, in addition to their normal function of being an indication and illumination device, LEDs can be used as a communication device for the transmission and broadcasting of information and data. Hence, it becomes part of a wireless optical communication system. It is essentially a new kind of short-range beacon to support vehicle-to-roadside communications. This allows a dual use of traffic lights because it can broadcast local traffic information, vehicle location and navigation information, and simultaneously perform its normal function of being a traffic signaling device. The objective of this technical paper is to present the results from an experiment of using traffic lights as a location beacon, and the bit error rate measurements in an indoor environment. INTRODUCTION As LEDs are increasingly being used in traffic lights or any traffic signal devices, their use for broadcasting vehicle location and useful navigation information to the driver will be presented. This development could lead to a new generation of such equipment because they can be used for the broadcasting of information and data, in addition to their usual function of providing the signaling, message display or illumination. This is a novel use of LEDs and extends from the work in the area of wireless infrared communications. This paper is based on the idea of fast switching of LEDs and the modulation of the visible light for communications. The main theme in this paper is on presenting recent results from an experiment and simulation of using traffic lights as a location beacon. Also, the result from a bit error rate (BER) measurement of an LED traffic light in an indoor environment is given. Prototypes for industrial use have been developed.
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BACKGROUND One important characteristic of LEDs is that they are semiconductor devices and are capable of fast switching with the addition of appropriate electronics. This idea has led to a number of patents [1-4] and this paper is a further development from the previous work. This paper is based on the use of visible light waves from LEDs as a medium for wireless, short-range optical communication. We have concentrated on the application in traffic lights or other related display boards [5-7]. LEDs are increasingly being used in traffic lights [8,9], message display boards, traffic signal devices, street lights or any other means of illumination. This paper extends from the work in the area of wireless infrared communications [10]. SHORT-RANGE BEACONS Short-range beacons is an effective way to support vehicle-to-roadside communications [11,12,13]. They have been used for both one way broadcasting, and two-way point-topoint communications. Their applications in ITS include traffic information broadcasting, vehicle location and navigation, automatic toll collection, vehicle identification, traffic management and vehicle-to-vehicle communications. The communication medium being used nowadays are either infrared or microwave. Infrared beacons operate at wavelengths of 850 and 950 nm and microwave beacons operate in the 2.5-and 5.8-GHz bands. Those beacon heads are usually mounted on the road facilities such as the poles of the traffic lights or some sign posts. They could be installed at every road intersection, with one or more beacon heads pointing at every direction of incoming traffic. The transmission data rate is usually around 500 Kbps with a bit error rate (BER) of 10e-7. One communication protocols being used is the dedicated short-range communication (DSRC) [14], which is the standard proposed by the European Committee for Standardization (CEN) and the American Society for Testing and Materials (ASTM). Some performance evaluations have been carried out [15]. Another standard being used is proposed by the Infrared Data Association [16]. Compared with the cellular and the radio data systems, beacon communications in ITS can provide a reasonably high transmission rate, good vehicle location, local area traffic information, as well as a means to accurately detect and measure the parameters of vehicles on a specific road or in a specific lane. This technical paper is based on the idea of developing an optical communication system from a novel use of traffic lights. A conventional traffic light at a road junction would provide the signaling for traffic control only. Due to the superiority of LEDs over incandescent lights, the traffic lights in the future could all be replaced by LEDs. Therefore, with the addition of a modulator, the traffic lights can be used as a communication device at the road junction. Like a beacon head for short-range wireless communication in many ITS applications, it can support communication between roadside and vehicles.
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INTELLIGENT TRAFFIC LIGHT The dual use of LEDs for both signaling and communication leads to a novel kind of traffic light. In addition to the normal function of providing the driver with the control signal, the traffic light acts as a short-range beacon and provides the driver with useful traffic information. It can provide the vehicle’s location and some useful guidance information (see figure 1). If the vehicle is equipped with a navigation system, the location can be identified on the map coordinates very precisely, with road segment heading. When used in conjunction with a dead-reckoning navigation device (a directional and distance sensor), the navigation system can determine the present location in real time and corrects any error when a location signal is received from an ‘’intelligent’’ traffic light. As seen in figure 1, it can also direct the driver to the nearest car park, hospital and gasoline station as well as giving traffic warning messages. Audio messages, which can provide further assistance to the driver, can also be received.
Figure 1 : Short-range beacon communication. IN-VEHICLE INFORMATION DISPLAY The developed methodology has the properties (and advantages) of a typical optical system: high bandwidth, free from regulation, potential low cost, and rapid deployment. With the change of traffic lights into LEDs in the future, only with the addition of some electronics, all the traffic lights can act as a roadside beacon. We have designed and developed a userinterface for the display of information and vehicle location received from a traffic light. Such an interface can form the front-end of an in-vehicle navigation system. FIELD TEST AND FINDINGS
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A prototype of an intelligent traffic light presented in this paper has already been built. This novel idea on the dual use of LEDs for both signaling and communications has been put into practical use. Audio broadcasting and data communication can be carried out. Careful design of the optical and communication systems are been carried out. Experimented results have been obtained. We have performed bit error rate test under different ambient environments and using different frame size for communications. We have also tested the system over a distance of around 40 feet with satisfactory results.
FIELD TEST A field test has been done in order to evaluate the effect of traffic light beacon signal on an invehicle system. Positioning data obtained by an in-vehicle positioning system was recorded by a mobile personal computer while the vehicle was driven on the street. The in-vehicle positioning system consists of a Rockwell GPS receiver, a Trimble differential GPS signal receiver and a Precision digital compass. The DGPS is used to find the distance traveled by the vehicle instead of finding the position directly. The traveled distance was found by distance readings between consecutive positions obtained from the DGPS. Angles turned by the vehicle were found by the digital compass. With the distance and angular information, positions of the vehicle could then be calculated which is shown in Figure 2. d2 (x2,y2) d0
(x0,y0)
(x1,y1)
d1
θ2
θ1
θ0
Fig.2 Dead reckoning positioning The above setup was used to simulate a dead reckoning system. However, errors in the distance measurement were accumulated as time went by which caused error in the vehicle position. Fortunately, this error could be reduced by updating the position by the LED traffic light signal. In our simulation, LED traffic lights were assumed to be situated at known positions on a map. The position of the vehicle is updated when it was within 20 meters of a traffic light. Figure 3 shows the results obtained in this simulation.
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Figure. 3 Positioning results with and without traffic lights correction
Key:
‘+’ – Positions without traffic lights correction ‘ ’ – Positions with traffic lights correction --Assumed traffic lights positions
As seen from figure 3, the positioning results with traffic light correction are more accurate than that without traffic light correction. Most of the positions with traffic light update fall within the road while most of the positions without traffic light update fall outside the road. Moreover, the effect of traffic light correction could be seen by the vehicle positions before and after the traffic light. The vehicle was traveling from upper left corner. Before the vehicle past the traffic light, the vehicle positions fell at the edge of the road. However, the car should be within the road. After the vehicle has passed the traffic light, the vehicle positions fell well within the road which are more sensible. This evaluation shows that traffic light corrections could reduce accumulated errors in a dead reckoning positioning system. BIT ERROR RATE A bit error rate experiment for the LED traffic light has been performed. The experimental setup is shown in figure 4. In the experiment, frames of data was transmitted continuously from a computer to the serial communication interface circuit via the printer port of the computer. The modulated signal is transmitted by the LED traffic light. The visible light signal was transmitted to the receiver and the serial communication interface performs demodulation of the data. The computer at the receiver side would compare the received data with the transmitted data. The number of error bits would be recorded.
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Receiver
Serial Communication Interface
Serial Communication Interface 12.8 meters
Figure 4 Bit Error Rate Experiment Setup The data frames transmitted by the LED traffic light contains pseudo-random series of data which is divided into 31 data blocks. The transmission speed of the visible light communication channel is 128 kbps. The indoor ambient light power was measured by an optical power meter which was found to be 12 µW. Table 1 shows the results of the bit error rate test. Power of visible light signal at the receiver 0.5 µW 0.6 µW 0.7 µW 0.8 µW 1.8 µW
Bit Error Rate 2.1315 x 10-3 5.2177 x 10-7 2.4835 x 10-7 9.1982 x 10-8 < 2.2155 x 10-10
Table 1 Bit error rate experiment results It is observed that the bit error rate decreases as the power (or luminance) transmitted by the LED traffic light increases. In other words, the brighter the traffic light, the less the bit error rate occurred at the receiver. In a real situation, there will be other visible light sources nearby. One example would be the head light of a vehicle traveling in the opposite lane. Thus, the effect of head light was evaluated. This situation was simulated by placing a lamp with 100W light bulb besides the LED traffic light. The bit error rates for the traffic light signal at 0.7 µW were compared. Without the head light, the bit error rate is 2.4835 x 10-7. With the headlight, the bit error rate is 1.1232 x 10-6. This implies that the head light may increase the bit error rate.
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CONCLUSIONS Traffic lights, traffic signal devices, message display boards are being replaced by Light Emitting Diodes (LEDs). This is due to their high-brightness, long life expectancy, high tolerance to humidity, low power consumption and minimal heat generation. In addition to their normal function of being an indication and illumination device, LEDs can be used as a communication device for the transmission and broadcasting of information and data. Hence, it becomes part of a wireless optical communication system. It is essentially a new kind of short-range beacon to support vehicle-to-roadside communications. This paper has presented on the development work from this dual use of traffic lights because it can broadcast local traffic information, vehicle location and navigation information, in addition to its normal function of being a traffic signaling device. The simulation study shows that traffic light corrections could reduce accumulated errors in a dead reckoning positioning system. The bit error rate (BER) measurements show that the BER is comparable with those using infra-red beacons. Thus, there is a potential for the wide-spread use of LED traffic lights for vehicle location and navigation.
ACKNOWLEDGEMENTS The authors would like to thank Professor E.Yang for sharing the original idea, and Drs. S.W. Cheung, P. Kwok and K.W. Tse for their discussions in meetings.
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